1. Field of the Invention
The present invention relates to methods and structure for measuring electrical or magnetic fields, and in particular to a method and apparatus for measuring high-frequency (HF) alternating fields in a nuclear magnetic resonance device.
2. Description of the Prior Art
It is known that water-bonded hydrogen nuclei (protons) of an examination subject can be excited so as to precess from a preferred or equilibrium direction imposed by a fundamental magnetic field having a high static field strength. The nuclei are caused to precess by the application of high-frequency (HF) excitation pulses. After the end of an excitation pulse, the nuclei precess with a frequency dependent upon the strength of the fundamental field, and return to the equilibrium position due to their spin after a predetermined relaxation time. By computation or by direct measurement of the integral proton signals, an image can be produced from the three-dimensional spin density, or from the distribution of the relaxation times within a slice of the examination subject. Identification of a nuclear magnetic resonance signal resulting from this precessional motion with the location of its generation is undertaken by applying linear field gradients. These gradient fields are superimposed on the fundamental field, and are individually controlled such that excitation of the protons takes place only in the slice of interest. Such image presentation is known under various techniques such as NMR (nuclear magnetic resonance) tomography, Zeugmatography, spin imaging, or spin mapping.
The quality of the slice images produced is essentially determined by the signal-to-noise ratio of the induced nuclear magnetic resonance signal. Because this signal is in turn dependent on the fundamental field, and increases with frequency, high-frequency signals are desired given high-strength fundamental fields. For example, substantially uniform high-frequency magnetic fields having a high signal-to-noise ratio for signal excitation and reception can be generated by HF coils. The coils have a shared sheath of material having good electrical conductivity which is non-transmissive for high-frequencies and transmissive for low-frequencies. Fields oscillating in equiphase fashion arise in the full volume enclosed by the HF coils, within which the examination subject is disposed. Such an arrangement is described, for example, in German OS No. 31 33 432.
A requirement for this purpose, however, is that the high-frequency magnetic field be homogeneous. The existing high-frequency magnetic field must therefore be measured in advance with a measuring probe, the probe being used to measure the field at different locations so that the field strength dependent on the probe position can be determined. To limit measuring errors, the high-frequency magentic field should not be disturbed by the measuring instrument. The surface area of the metallic components of the measuring instrument must therefore be maintained small. In particular, the mechanical mount of the probe cannot contain any metallic carrier elements and cannot contain a metallic electrical line, for example a coaxial cable.
Probes for measuring electrical or magnetic fields in other environments are known wherein the mechanical mount is a rigid coaxial cable, which simultaneously serves as the electrical signal line. Due to the necessarily large longitudinal extent of such a metallic mount or signal line, the introduction of this probe into the field to be measured disturbes the high-frequency electromagnetic field.
Probes for measuring local antenna fields are also known wherein the signal read-out is made optically through optical fibers, as described in "British Labs Target Enhanced EMC Tests," Mitchell, Microwaves and RF, June 1985, pages 51-55. In order to convert the measured electrical signals into optical signals, however, a relatively complicated interface having an integrated power supply is required. The metallic extent thereof amounts to a number of centimeters in at least one direction. Such an optical signal read-out arrangement is thus unsuitable, for example, for nuclear magnetic resonance measurements.